1. Field of the Invention
The invention relates to a high pressure mercury lamp, especially to a super-high pressure mercury lamp of the short arc type in which a discharge vessel is filled with at least 0.15 mg/mm3 mercury and in which the mercury vapor pressure in operation is at least equal to 150 atm.
2. Description of the Prior Art
In a projector device of the light projection type, there is a demand for illumination of the image uniformly onto a rectangular screen, and furthermore, with sufficient color reproduction. Thus, the light source is a metal halide lamp which is filled with mercury and a metal halide. Furthermore, recently, smaller and smaller metal halide lamps, and more and more often, spot light sources have been produced, and lamps with extremely small distances between the electrodes are used in practice.
Against this background, recently, instead of metal halide lamps, lamps with an extremely high mercury vapor pressure, for example, with a pressure greater than or equal to 200 bar (roughly 197 atm) have been proposed. Here, the increased mercury vapor pressure suppresses broadening of the arc and an extensive increase of the light intensity is desired; this is disclosed in Japanese patent disclosure document HEI 2-148561 (corresponding to U.S. Pat. No. 5,109,181) and Japanese patent disclosure document HEI 6-52830 (corresponding to U.S. Pat. No. 5,497,049).
In a light source device which is used for such a projector device, in conjunction with projection of a clear image, it is considered very disadvantageous that the discharge lamp devitrifies. On the other hand, recently, the DLP (digital light processor) method using MMD (micro mirror device) has been used, by which a liquid crystal cell need no longer be used. This yields smaller and smaller projector devices. This means that, in a discharge lamp for a projector device, there is, on the one hand, a need for high light intensity and high maintenance of illuminance, and on the other hand, a need for a smaller discharge lamp according to the reduction in size of the projector device and for stricter and stricter operating conditions.
The material of the discharge vessel with respect to the UV light transmission property is generally quartz glass. However, there are cases in which a residual stress is produced in the quartz glass during the lamp production steps. This residual stress influences the high light intensity and a high degree of maintenance of the illuminance of the discharge lamp. In conventional lamp production processes, to eliminate or reduce this residual stress, the discharge vessel is subjected to high temperature heat treatment (annealing).
Furthermore, besides eliminating the residual stress in the quartz glass, there is also the technique of controlling the crystal structure in itself. This technique is based on the idea of not removing the residual stress which has formed, but devising a quartz glass in which no stress occurs as a result of its nature. This control of the crystal structure specifically means control of the fictive temperature. It is known that devitrification of the quartz glass can be effectively reduced by using this technique. One such technique is disclosed, for example, in Japanese patent disclosure document HEI 7-215731.
However, from an operating test performed with a discharge lamp based on the technique disclosed in the above described Japanese patent disclosure document HEI 7-215731 used as the light source of a projector device, it was found that, in practice, advantageous operation cannot always be carried out. Specifically, the discharge vessel is devitrified in the course of operation of the discharge lamp, by which the degree of maintenance of the illuminance decreases or by which damage, such as cracks or the like, occur in the discharge vessel. On the experimental level, there are also serious cases in which the discharge vessel is destroyed by these cracks.
The primary object of the present invention is to devise a super-high pressure mercury lamp for a projector device in which a discharge vessel made of quartz glass is filled with at least 0.15 mg/mm3 of mercury and which has a new arrangement in which both devitrification as well as damage of the discharge vessel can be eliminated.
The object is achieved, in accordance with the invention, in a super-high pressure mercury lamp in which there are a pair of electrodes opposite one another in the quartz glass discharge vessel and in which this discharge vessel is filled with at least 0.15 mg/mm3 of mercury, by the above described quartz glass having a fictive temperature of 11000xc2x0 C. to 1250xc2x0 C. and moreover, by the total content of alkali metals being from 0.1 ppm by weight (wt) to 3 ppm by weight (wt) and the aluminum content being from 1 ppm by weight (wt) to 30 ppm by weight (wt).
As a result of careful observation, to achieve the aforementioned object, the inventors noted that, in a super-high pressure mercury lamp for a projector device in which a discharge vessel is filled with an amount of mercury that is greater than or equal to 0.15 mg/mm3 and a halogen gas, neither devitrification nor damage to the discharge vessel can be eliminated solely by controlling the fictive temperature (crystal temperature) of the quartz glass. They have considered the circumstance that the internal lamp pressure (mercury vapor pressure) during operation is extremely high, and have found that to eliminate this defect it is a good idea, in addition to controlling the fictive temperature of the quartz glass, to fix the total content of alkali metals and the content of aluminum which are contained in the quartz glass.
In the above described citation (JP-OS HEI 7-215731) in which the fictive temperature is fixed, there is, in passing, a description of use an excimer lamp and the like for a high pressure mercury lamp. However, the actual description presupposes a low pressure mercury lamp.
Furthermore, the invention relates, not to a general mercury lamp with a mercury vapor pressure during operation of at most 1 atm to 10 atm, but to a lamp filled with at least 0.15 mg/mm3 of mercury in which, during operation, a state with an extremely high pressure of at least 150 atm is produced. This lamp is an extremely small discharge lamp with an inside volume of the discharge vessel (inside volume of the discharge space) of, for example, at most 70 mm3, which has an operating state which is so different that it cannot be compared to a general high pressure mercury lamp.
In the discharge lamp of the above described citation, the fictive temperature is mentioned. However, this presupposes a low pressure mercury lamp. Assuming that an application for a high pressure mercury lamp is mentioned anyhow, this relates to an extremely general high pressure mercury lamp with a pressure of at most roughly 1 atm to 10 atm. The inventors have found that the same effect cannot always be obtained by simple use of the technique described therein for the high pressure mercury lamp according to the invention.
As a result of further detailed consideration, the inventors have also found that alkali metal elements (sodium, potassium and the like) which are found in quartz glass are inserted into the chemical bond of silicon (Si) and oxygen (O), which are components of quartz glass, that these alkali metals are influenced by the mercury and the halogen elements which are present in a large amount within the discharge vessel, and that, in this way, devitrification and damage to the discharge vessel are caused. The inventors have found that the above described adverse affect of the alkali metals can be prevented by mixing aluminum into the quartz glass of which the discharge vessel is formed.
The invention is further described in further detail below with reference to the accompanying the drawings.